1. Field of the Invention
This invention relates to a watch case and more particularly to a watch case having a glass soldered to a metal part of the case, as for instance a bezel or a middle.
2. Prior Art
Such a watch case is for instance disclosed in European Patent Specification No. 0066538 which teaches applying, beneath the glass, over the area thereof intended to be soldered to the metal part of the case, a particular form of metal coating. This coating is made up of different superposed layers, i.e. a first, masking, layer, e.g. of gold, chromium or nickel, for safeguarding the aesthetic appearance of the case by masking the solder area, a second, barrier, layer, e.g. of chromium, for preventing the weld material from diffusing towards the glass and spoiling the appearance of the masking layer, and a third, final, layer, e.g. of copper or gold, for ensuring a proper bonding of the solder material to the coating thus formed beneath the glass.
Such a structure has proved to be particularly effective in the production of aesthetic watch cases involving the soldering of a glass at low temperature to a metal part. By low temperature soldering is meant soldering resorting to the use of a filler material, e.g. a silver or tin alloy, having a melting temperature below 550.degree. C. and preferably between 180.degree. and 250.degree. C. The mechanical resistance of such assemblies is excellent, the wrenching force being in the region of 6 kg/mm.sup.2.
However, in some applications an even better mechanical resistance is required, i.e. a wrenching force of 15 kg/mm.sup.2 is more. In such cases the assemblies necessitate high temperature soldering, i.e. involving the use of a filler material having a high melting point, e.g. 600.degree. C. or more, of the kind commonly used in, for instance, the soldering of steel parts. To produce such assemblies, coatings of the kind set forth above are no longer appropriate, in particular because of the strains caused by the high temperature reached during soldering.